Ummmm.... I wonder. If it's really reasonably pure U-238, it won't be very radioactive - not at all useful for a "dirty bomb." If one went off near me, as long as I was outside the blast zone I'd probably get more radiation from the bricks in my house than from the U-238. For that matter we'd probably get more radiation from the C-14 in our bodies than we would from such a bomb.
If it's got significant radioactive impurities then all bets are off but it will be those impurities that are generating the radiation, not the U-238. For example, spent nuclear fuel rods, whose high radiation content is primarily due to the fission products in the rod, which have a very short half-life and give off a LOT of radiation.
Mentioning U-238 without saying whether any other radioisotopes might be present is a pure red herring. The question is whether the misunderstanding was in al Qaeda (thinking that their bomb would be dangerous if they just used some kind of Uranium) or with the reporter.
US President Bush seems to have confirmed this, not in specific words,
but by blocking the assets of UTN. UTN, a non-governmental
organization was founded by Pakistani nuclear scientists that provided
information to Osama bin Laden and the Taliban about chemical,
biological, and nuclear weapons.
Bashir-ud-Din Mahmood, who was formerly the director for nuclear
power at the Pakistani Atomic Energy <<<<
The above makes me think that he had the needed information,
from the scientists to make anything he wanted.
Who knows what was in the barrels that they talk about.
I wanted a rock house, until I learned about the natural
radiation in them, in this area.....there are very few of them here.
It is difficult to say what al Qaeda has, but my guess is the finest that money will buy, so I expect them to do anything they decide to do.
As to how it works, that I am not educated to do.
Never far from my mind is the fact that so many of them have been trained in our finest universities.